Swift observations of the X-ray - Bright GRB 050315

This paper discusses Swift observations of the γ-ray burst GRB 050315 (z = 1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ∼200 s and several breaks. However, both the prompt hard X-ray/γ-ray emission (observed by the BAT) and the first ∼300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests that the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt γ-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray - rich GRB. The prompt (BAT) spectrum was steep down to ∼15 keV and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline, the X-ray afterglow did not fade for ∼1.2 × 10⁴ s, after which time it decayed with a temporal index of α ≈ 0.7, followed by a second break at ∼2.5 × 10⁵ s to a slope of α ∼ 2. The apparent "plateau" in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break, then the jet opening angle was θ₀ ∼ 5°, implying Eγ ≳ 10⁵⁰ ergs.